Consider statements about cell biology: 1. The plasma membrane is composed of a phospholipid bilayer. 2. The nucleus contains the cell's genetic material. 3. Mitochondria are the site of photosynthesis. Which are correct? MCQ with Answer and Explanation

Explanation:
The Lac operon in E. coli is a classic example of an inducible operon. It is normally 'off' (repressed) because a repressor protein binds to the operator, preventing transcription. However, when the inducer (allolactose, derived from lactose) is present, it binds to the repressor, inactivating it and allowing RNA polymerase to transcribe the genes needed for lactose metabolism.

Explanation:
The pineal gland is a small, pinecone-shaped gland located in the epithalamus, near the center of the brain. In humans, it is not a true eye, but it is photosensitive; it receives information about the light-dark cycle from the retina via the suprachiasmatic nucleus and the sympathetic nervous system, and uses this to regulate the secretion of melatonin, thus controlling circadian rhythms. In some lower vertebrates (like certain reptiles, amphibians, and fish), the pineal complex includes a parietal eye (or 'third eye'), a light-sensitive organ on the top of the head that can literally detect light and shadows, helping with camouflage and predator detection.

Explanation:
Robert Hooke observed cork tissue in 1665 and coined the term 'cell' to describe the box-like structures. Matthias Schleiden, a botanist, concluded in 1838 that all plants are composed of cells, forming a core tenet of the cell theory. The cell nucleus was discovered by Robert Brown in 1831, not Antonie van Leeuwenhoek, who is famous for discovering living microorganisms. Thus, statements 1 and 3 are correct.